Heart pacemaker systems have been operating on the basis of detecting the electrical activity of the heart and adjusting the pulse generator output accordingly. More sophisticated pacemaker systems require that more information is obtained about the status of the heart, particularly in relation to the mechanical/hemodynamic activity, to more accurately simulate a natural, non-diseased heart. Sophisticated pacemaker systems are required to pace at different rates, determine the minimum amount of energy required to stimulate the heart and to provide back-up physiologic information to supplement the sensed electrical signal.
One way to minimize energy consumption of a pacing system is to use pulse energies that are as close as possible to actual tissue stimulation threshold requirements. To make a pacemaker system that can automatically adjust the pulse energy output necessitates the determination of the automatic stimulation threshold. This has been difficult to achieve as the measurement of the heart evoked potential following a pacemaker stimulus cannot be detected reliably due to electrical charge build up around the stimulating electrode immediately after pulse delivery. An alternate method to detect the heart capture is by a sensor that detects the mechanical/hemodynamic changes in the heart associated with a heart contraction.
Artificial Implantable Defibrillators (AID) also require the sensing of the hemodynamic state of the heart as electrical sensing alone may not be sufficiently reliable to distinguish between fibrillation and tachycardia.
Transducers for measuring various physical or chemical parameters from the heart, or other parts of the body, have long been a goal of pacemaker system designers. Up until now, these transducers have been affected by poor reliability and a great deal of complexity. Transducers to measure pressure within the body are known to the art from U.S. Pat. No. 4,023,562 Hynecek et al, which is incorporated herein by reference, but are typically only used acutely, such as for temporary diagnostic purposes.
A successful sensor should have the following requirements:
sufficient stability to give useful measurements over the life-span of the heart pacemaker system; PA1 a size small enough to be compatible with the heart pacemaker system and also have minimal power consumption; PA1 packaging of the sensor in such a way that toxicity or other undesirable effects such as thrombosis or mechanical limitation are avoided; PA1 compatibility with the rest of the hardware system; PA1 an output signal which can be readily processed by the pacemaker's software system (where fitted); and PA1 an output signal which provides useful physiological data.
One attempt which has been made at producing a sensor which produces hemodynamic data in the form of pressure information and which meets some of these requirements is disclosed in U.S. Pat. No. 4,485,813 to Anderson et al., the disclosure of which is incorporated herein by reference. In this design a piezoelectric ceramic is used to convert pressure and motion into electrical signals. This design has the disadvantage that it required power from the pulse generator to make it function.